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A Word from the Koi Pond

By: Bob McKay, Pond Specialist and Gardener | 07/09/2019

Here at the Idaho Botanical Garden (IBG), the koi pond has had thick blooms of suspended, single cell algae and long, stringy blanket weed or “hair” algae. As IBG’s Pond Specialist, I deal with algae using a combination of solutions—none of which is a “silver bullet”. All ponds must deal with algae at some point. So, let’s define algae that are typically seen in our koi pond.
String Algae

AKA: Filamentous algae. Mat algae. Claudophora spirogyra.

String algae provide a habitat for numerous aquatic animals as well as food for some. Koi typically don’t eat much of it. Once you have these type of algae, daily efforts of mechanically pulling algae out of the pond become routine until some sort of control is employed.

Causes:

· An abundance of nitrate and phosphorus

· Too much sun exposure

· Too much uneaten food

· A lack of oxygen in the water

· Requires a current in the pond to grow

Suspended Algae

AKA: Planktonic algae, diatoms, blue-green algae, green algae, pea soup.

These algae are single celled, free floating and often found in the top few feet of the surface of the pond. In a natural pond, a greenish coloration can indicate ample food for small organisms, that in turn support a higher fish population. When suspended algae levels are too high, this can lead to very low pre-dawn oxygen levels or Harmful Algae Blooms (HABs).

Causes:

· An abundance of nitrate and phosphorus

· Too much sun exposure

· Too much uneaten food

· A lack of oxygen in the water

Some algae are not a bad thing! Too much algae complicate pond care and make it hard to see those koi you spend entirely too much money on not to see.

Algae Management

Prevention is in the planning for the pond build:

· Don’t build your pond in unremitting sunlight

· Consider ultraviolet treatment

· Consider elevation and aspect: yard water runoff should not end up in the pond

· Properly size your pond for desired fish load, water volume, filtration capability, and oxygenation not to mention properly sized pumps for better than adequate water movement

· Provide adequate circulation in the pond, horizontally and vertically, so there are no still areas

After the pond is built, adjusting the capabilities of the installation will help:

· Add a bigger air pump and/or water pump

· Increase water changes

· Add UV lamps

· Improve filtration capability

· Install shade cloths or add trees

· Feed fish less or keep less fish

· Use additives such as Barley Straw Extract

Here at the Garden, the koi pond was constructed in a site that has good portions of sun and shade throughout the day. I use UV lamps, Barley Straw extract and High Fin Sharks as a work crew. I monitor water chemistry closely so that I can adjust how I feed the fish, and do weekly water changes.

Every pond is different and will require time and some trial and error to discover the best course of action to control algae.

Bob McKay has been Idaho Botanical Garden’s Pond Specialist and Gardener since 2016. He is an Internationally Certified Koi Keeper. Idaho Botanical Garden is a member of the Idaho Water Garden and Koi Society, and hosts the annual Koi Show, this year on Saturday August 31st.